Arduino fried on 12VDC? Only 'ON' LED turns on when connected with usb

Hello,

I was using a 12VDC power supply to power my arduino UNO. Suddenly everything stopped, and since the voltage regulators were kinda hot i removed power. After a day I connected it via USB to my computer, with [u]nothing[/u] connected. All that happened was the green 'ON' led turning on. Could I have fried my Arduino?

Details:

Was using 3 74HC595 shift registers to power 20 LEDs. Everything had been working fine the day before for at least an hour straight. Today, though, it 'crashed' after about a minute.

When i connect Arduino to my PC, it doesn't recognise it (no tudu sound (windows) and I can't upload a sketch or open the serial monitor either).

This is now a useful exercise in the hardware side of "debugging".

You do have a multimeter, do you not?

Start by measuring voltages with it plugged into the USB port of the computer, see if 5V (or something near) is available at the Vcc terminal and if not, where it is or is not, such as before and after the polyfuse (refer to the schematic).

To see what may be damaged, carefully remove the Mega328 and test without it. Presuming you still have 5V supply if it is not recognised by the PC, that indicates damage to the 16U2 which would indeed be difficult to replace, but if it is simply one of the power components, that should be repairable. And there are various "tricks" - if for example, the regulator is dead, it can be removed and the board operated with USB or 5V regulated power via Vcc.